An operation is described in which approximately 100 sq. miles of heavily infested tsetse country in Southern Rhodesia, supporting both Glossina morsitans Westw. and G. pallidipes Aust. and an abundant and varied game population and formerly an area of endemic sleeping sickness attributable to Trypanosoma rhodesicnse, received six applications of 4 per cent, γ BHC in diesolene, discharged from aircraft in the form of a coarse aerosol, between May and September 1957. The area was a valley bounded by an escarpment, which provided conditions of relative isolation.

Fly-round catches of G. morsitans taken before, during and after the operation indicated that in those parts of the area that had received uniform treatment an estimated reduction in population, after the final application, of between 96·6 and 100 per cent, had been achieved, but where only the vegetation forming the riverine fringes had been treated the reduction was less. Quantitative data on the effect of the treatment on G. pallidipes were not collected, but the species ceased to be found in an investigation on sampling techniques that had been in progress in the area.

The area had been chosen for a resettlement scheme of indigenous people that was due to begin before October 1957, and the purpose of the spraying operation was to reduce the population of tsetses to a level that would permit the introduction of the people and their stock with comparative safety from trypanosomiasis. It was hoped that the number of people to be resettled would be sufficient to clear most of the tsetse habitat in their text-abstract agricultural activities and that the tsetses would be permanently excluded from the valley. Four to five years after the start of resettlement there were still several uncleared areas remaining, but the situation was continually improving, and estimates in 1962 indicated that the population of G. morsitans had remained at a low level, showing a reduction of between 81 and 100 per cent, as compared with that prevailing before the spraying operation. The stock introduced in 1957 were maintained under drug treatment, and the rate of infection with T. congolense was kept below a 3 per cent, level; no cattle died from trypanosomiasis, and their numbers had more than doubled by 1962. No case of sleeping sickness occurred.

The work described here showed that, under conditions of relative isolation, G. morsitans can be sufficiently reduced in numbers by means of an insecticidal aerosol discharged from aircraft to permit the successful introduction of people and stock, provided that adequate veterinary supervision is given, and that under these conditions settlement can prevent the recovery of a population of this species.